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BSTJ 60: 7. September 1981: Digital Signal Processor: Voice-Frequency Transmission Treatment for Special-Service Telephone Circuits. (Blake, R.B.; Bolling, A.C.; Farah, R.L.) PDF

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Preview BSTJ 60: 7. September 1981: Digital Signal Processor: Voice-Frequency Transmission Treatment for Special-Service Telephone Circuits. (Blake, R.B.; Bolling, A.C.; Farah, R.L.)

Digitat Signal Processor: Voice-Frequency Transmission Treatment for Special-Service Telephone Circuit By A. 8. BLAKE, A.C. BOLLING, and RL. FARA (Manuscript recetved Jury 30, 1980) This article describes the application ofthe digital signal processar ea voiceband signal processing element, The application choven it one of the most stringent noire frequency signal procesting appa tions in the telephone network providing transmissian treatment (em, gain, equalisation, and echo control jor special aervice circuits A vetted description of a protatype transmission tratisent uni, ‘which uses dhe Dap. ie provided along neh descriptions of the digital Fitter structures and filter ayntheots techniques. Measured results for repreventatice extreme cable facility cases are presented, showing that digital signal processing utilizing the use meets the telephone network transmission requirements fur ypecial sevice cirewts, 1. wTRODUCTION ‘The frst, and still most heavily «sed, (ransmission medium in the felecammunications network is copper wire, Twisted pair metalic cables of various gauges, lengths, and sees make up the bulk of the loop plant and local exchange trunk plant. The pplietion of electronic amplifies or repeaters to provide gain to compensate for Ihe wen {ation of signals on mcallio eables ws the ft large-scale use sleccronies in the celecommunicaLions network, Repeaters ate sil sed extensively. By necessity, che systems of the pase chat proved voice-requency (vr) transmission irealmenl on 8 per-channel Basis ‘were analog aystems, The lange-acale use uf sophistented dig signal Drocedsing for these sransmiseion treatment functions was prelate By cos, power consumption, al ste of amplementation. However, in recent years the capabilites uf digital hardwaze have iproved nificantly, primily ms result of the rapid development of integrated ‘Sreut technology. "ha digital signal procesor (nse, a vist device with large umber oflogie erie, ivan example of dhe sophistication that is now posse ‘vith digital hardware, The inherent capabilities of the ost have made it posible to consider the se of digital flters to replace some of the ‘raditionl analog network functions in vr transmission systems. This per reports on the results af an experimental try in which he one fn sed for this purpose. Amung the ost desirable features of the sr, and sqpecaly important for this study, isthe ease with which ite be programmed under compuler ennrol to provide transmission tre ment functions, "The principal tansmision signal pracesing fonctions are performed in ve repenters for metallic tenamission systems and in carrier ler ‘minal units (Crus) fr tranemieson ayers Chat contain a metali-to- ‘arror interface. Taste syseraa provide signal processing fora variety of services Typically, the most demanding sxnal processing require- ‘ments derive from special services applientinn" Special aervice eteuits fae enyincered using all pes of transmission medie. Special services fre large and rapidly growing pare of the ‘clecommunieations net- ror Performance objectives for a special service creuit are normally spetified in terms of kHz oes attenuation distorion, eco dacorion, nd various oer teanemiesion parmmeters. 1 signal proceasing faust The ficluded in a eieuie £0 that {ranisson objectives for ehe creat fan be achieved, che eicuit is called » treated cireuit. ‘The prototype ‘ananiasion teatment unit, doscrhed in thi article, whieh uses the Dro provide adjustable tranenision treatment fanetion, is referred tos digital resent transmission amit (0°70). The description of the v7 and ite capabilites gine in Section LL Fir, in Section I “nn oulline of treated circuits is prevented sinc the performance goals for the DTT were based on the performance abjoctives for these circuits 1. INTRODUCTION To VOICE-FREQUENCY CIRCUITS “Table Llsts some ofthe applications for metallic ceatment systems Figure 1 llustrates some of the possible circuit arrangements with tangas treatment, Each of the arrangements is assumed to be providing a special servi cicuit—in this ene « foreign exchange trunk. Figure la shows termingl repencr, with a switching system. timane side and a2-vir ele connected to a PRK on the other. Figure This amila, but with the repeater placed at an intermediate location inthe circuit Figure Te sows the use of to repeaters na long cuit. Finally, Pig, 1d shows 2 cteut that contain both aeazter lin and 1886 THE BELL GYSTEM TECHNICAL JOURNAL, SEPTEMBER 1851, Table ImInserted Connection Lows 1) objectives for typical 2-ire itched special services: eneee| ee 1 Devo cab facility. This latter configuration, with a earir link in terudem with a 2-wire metallic estension, ix quite commen, Cartier systems are designed to be ersentilly (rparent for transmission ppurpmes Noto cht ifthe carrer Ink is removed in Pig. 2d end the rvs retained, the rolling circuit amangenent is xinilar to that shown in Fig. 1a, "The crv that provide the 4- to 2wite interfaces must provide VOICE-TREQUENCY TRANSMIGSION 1587 eanamission ereutment. fir ehe adjacent metallic cable. Carrer termi dl unite with analog reatment ze currently available for both iit nd. analog carrier syst, A digital carrier ayetem is un obvious Candide for dil traneriaion treatment since digital unending and tleooding of signals ie alrendy required. Digital switching. systems, ‘whieh have transtission characteristic similar to dial carsees are lo strong camfidaten for digital treatment of special service circuits "This article concentrates on the digital to-analog 4 2wine incertae. 2.1. Tranamiesion veatmint objectives "The auecess of the irlecontununiestions nevwurk in providing atis- factuty service places requirementa or objectives on some ofthe base tlt ircal charaereistios nf the equipment wit. The service sted in ‘Table Tort be wngineeted to have earehlly vontolled ansmission properties ‘The mnwin theme of this study i the use of the nse to Eontol the loss attenuation dislrlin, and echo distortion of imate Clreuile. This iv accomplished hy raplerarntng digital fers with the Da. as is described in detail in tho following sections. AlUhongh not Considered here, the osv could wbo be programmed to contr elay Aistoriion of x treated acu. ‘Ofte service Hsted in Table , che trunk have dhe most stringent performance objectives A typical shore bal treed special service Trunk must be engineed to have a LAH los of 35 dB to en noeuracy bt approsimately 0.8 #B, Furtheraore, The lose of the eireit at <i) tod 200 Hi relative tothe 1-KHz les, should he within the fellawing limits: a 400 Ha, ~L0 dR wo 80 dl; at 2800 He, ~1.0 dB to 4. dB, Aulitionlly, the eireult Joa should be relatively smooth between ‘hone ovo froquoneis. The cieuit should ssn he designed co chat its lose response rolls iff tthe vv band edges o enbancestaility margin sn echo performance. For hw eniguration shown in Fig. 1a, emstalh Considratinn retret the gin of the repeater to a maxim af 6B, 1 kM, and the loge ofthe able to a maximum of dB at 1 ki Since u 2-wire treatment unit is inherently a feedback device, some ‘means rust be provided to “balance” the unit, tha i, significantly Fede che feedback, io ensure adequate stalihiy margin and satistac tory echo performance for a treater cirmit. Tn the DITU, a digital Faneeler network provides this balance furetion. Since gain adel 10 flreut amplifisan echo along with the denzedsigasl, erento circuits fost be beter balanced shan unleeated cizcuits. Poor lar ests in listener distortion on short civics end caller echo on lng eieuita. ‘Ava 4 to -wie interface, Uhe equipment should produce atleast 15 fob a8 of los between the 4-vire porta (eg, porte sand 2 Fig. 21, Incuding Un: eects of gun, over most of che voiceband. The Ins at the vv band edges may be somewhat les, although adequate stables ‘margin mast be rineaine. 11680 THE RELL GYSTEM TECHNICAL JOURNAL, SEPTEVBER 198 [Oo "eine Poly oir a ache fle ie 159 42 as beren 1M, DIGITAL TREATMENT TRANSMISSION UNIT A\ bloc diagram of the D770 is shown in Fig. 3. "The digital ports of the ovre eval interface witha digital erser oF a digital switch, while the 2owire analog por: could interface with logp plant cable or trunk ‘able. The DTTC consists of @ DSP, ine interface unit (170), nel the “approprinte lgic devices to interface the os tothe 10. All xjuntable {in, equalization, and echo cancellation are provided by digital filers lmplemented with the nse. The remaining DTT functions, auch an ‘encoding and decoding of signals are proved in the Li. 3.1 Line intertsce unt ranamision functions The experimental version ofthe LIU contain a commercially aval able 16 fal Knenrdigital-n-analogcouvertar, Unpublished wuicn & 4 YOICE-FREGUENCY TRANSMISSION 1860 by J.J AW. Unger have sown thet she standard 1-255 encoding cnes ‘not pride sufficlene dynamic range for digital signal proccss ome special service appliations. The 16-bit hinear enced ‘hove to avoid this problem, ‘The convertor ie followed by a recon “ruction filter that removes energy above 4 KEfz. The reennamuction filter driven a fixed-pain anslog amplifier that, in turn, dives the 2- cable throngh a M-ohm teansformer-coupled output. Addition. the amplifier drives an analog compromise cancaler, a device fiscunsed in more detail inthe next paragraph. Input signals from the fable are coupled through the transformer lo a diferential ampliiee, ‘The eitferential amplifier drives an antialiasing ver that. removes feneris above 4 Kilz and sn removes 6-1 Induction. The oucput of the antialiasing iver is sampled at 8 KHz snd then concerted co digital form by ¢ 16-bit falinese analog-to dlgial converter. "The primary purpose ofthe compromise canceler isto provide some lose tappreximacely 6 aH or gresver) between points “a” and "Bi of the LT for the universe of cublen with which the prTU is expectei Interface, Is transfer function is Bxed, with one polo end no wos. AS compared 10 the ne performance in the abeence af the compromise tanceler, the henefits obtained are (wnfold Te large signal se point "ais let Hikely co overload the analog tondigial nvertor Tis The performance of the diyilal echo canceler in the DAP is enhanced. 2.2 Diptaeanalproceesartrenariasion functions As mentioned above, all adjustable gain, equalization, and echo cancellation for the D790 are provided by digital flere in the nse. ‘These fiters xe: 1 an equalize forthe transmit rection of transmissnn, (i) an equalizer forthe recive divection of transmission, and ia) a canceler, ‘The equalizers provide adjustable gain as woll ws adjustable equal zation. "The trefor functions of thebe three Miers are dentoed by E, 5, and C, respectively: There filvers wil be represented in many of the remaining figure in Cie artice bythe symbols shown in Hig 4 |W. SELECTION OF FILTER FORMS FOR THE EQUALIZERS AND "The neve must have fhe capeblity to provide tronsmicion tout sent for large varity of cable facilities, Laboratory and computer ‘Stuulationsf a representative aample of treated trannisionfacities Ihave ahown that the OTe hae the required capability if the eanceler su S2ap transversal Fler and each: equalizer is a fourth-order 1590 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBE 1201 Fig. «Syme rprsntain af re dtl er recursive filer composed of two cacaded biquads. In Z-transforms rotation, the Ger finetions are 0-8 me", omy Sachse tate yy ey Sets tae & TH b= Baie ‘Symbolic diagrams ofthese filters are shoven in Figs $ an 6. (ry the Figures, 5,7, and vs reprosont signal values, and are ced for the iscustion in Seetion V.) The equilizer premuluphers, Gy and Gr allow flesbility in parceling out yin belorden the ter sections and in ania VOICE-FRrQUENGY TRANEMISsICN 1504 controling corficiene magoituds, which is constrainei ly the se oeticient slorage format to be le than 20, ‘The coficienss in the thrve equations above ae different foreach differen eal fic. ‘A recursive sruciur, rather than transversal stritine, suse for the equalizers for one primary Yeasor: in addition vo providing gain fan etalization it the frequency bxevd 400 and 2900 Lx, for 2-wire ‘ranemiasion it volo necessary thatthe equalizers provide significant sttenuation athe ¥F band edges to enhance che stability margin pesformance ef the Dru, This charwolerstic i moze resdily obtained {witha low-order rsursive tract, fur he eensmission system under ‘onsderasion, than it is with a reasonable ize transvrratl er "A cneaded biquadstuotare rather than a dliret-form fourth-order saracue, is uso! for the equalizers became the transfer fanction the easeeded strucre is Int sensitive lo coelfiient quantization. In Feta, 4 and 6 i is show tha if the unquantized poly locations oF zee locations lle o each other, ceMicen- quantization can cause ‘tovge shift in the pole or taro leations, I Ue sitt occur, the filter fesponve wil differ from the desi response. In the direct-form Fealizstion. the shit as 4 funtion of eveficient quuntization is de pendent of sll the pole of zero locations. Howeee, in the cascade Form. the pole or zero aif i one section is indepen. ofthe pole or ev locas in the other sein \, FILTER INPLEMENTATION WITH THE OS? In this vetion, the implementation of the equalizers and canceler with che vot is disoused, Fora description of the nsP wehiteccure see [Ret 1. Thorn are four tops of nieve with regard to the nse program. ‘These ure 41592 THE BELL SYSTEM TECHWAL JOURNAL, SEPTEMBER 1981 1 Location of eoetvins andl daly ithe a0, (i) fer implementation, Ain) coetficen loaing, and lie) program storage ad exoewtion tin, 5.1 Digit! strat processor RAM mamory map Sinee the filter ooeficionts are different for each diferent. cable facility, the must be stored in the Bs Kaa, Ala, che delayed signal, luca must be sored i RAK. Coecients and delayed signal values fro scored a ahown in Fig. 790 a Wo minimive the number of reine ets vequized fo access them, ‘5.2 Filter implementation 5.2.1 Equalizers Tn the ust program used for the nr70, the three filtering operations are performed inthe folowing onder re Wa 6, Gin B ‘Thecanceler is dices below, in this section F, and Bare discos, ince the program stop used to implement and Bate identical, « seneraldesenption of the Gltering steps in to exaraded biquads i prowated, Figure 6 shows e block diagram of two cascaded biquads. The inpat signal is labeled and the output sigual is labeled y, Internal signal values are Inbeed ta andl with ef = 1, 24 — 2) etm — 2), nd 2.0 ~ 20 being delay signals obeained ftom preseling fer ope tions, The general sequence of eperations used ly fer and obesin requires nine additions snd eleven multiplications for uach value of inpat ta the ter Fire, y(n) ieobained by forming the sun re Wo8 29 + by own = 1D AG extn in the accumulator, rounding che eat, and aor i in & temporary register. "The round resul soul). Thw prodacis are performed the product registor before entering the werurnalator. obtained in an hlentical meaner from the sum tg #10 = BS ey ENF) + as 90) hag Hn 2) 4 bus in, Finally, fm} obtained from the rounded vale of the sums in = 1 ais et \VOICE-FAEQUENCY TRANSMISSION 1899 11] omen re ronconectcet EEEE Beer gtx the en od des a [Before the next valuw of x enters the filter, eo(a ~ 2) is updated by setting it waval roan ~ 1s wire ~ 5) ia updated with ae): tale — 2) swith evn Tis and ete — 1) with ri) “The outpaLof the receive path nqualizer is transmitted tn the digital to-enalog converter and tothe cancel input. 622 concolor Figure 5 shows a block digramn ofa 52¢ap transversal Mer. The Desie operation in transversal fuer che multiplication of a tap ty a delay ov‘put, followed by an addition in an sccura i operation ean be accomplished with « sequence of inscuc ‘aq THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1861

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